1. Field of the Invention
The present invention relates to a process for the disposal of low, medium, and high-boiling secondary components formed during the preparation of (meth)acrylic acid.
2. Description of the Background
Acrylic acid is an important basic chemical. By reason of its very reactive double bond and also its acid function it is particularly suitable for use as a monomer for the preparation of polymers. Most of the monomeric acrylic acid prepared is esterified to, say, adhesives, dispersions, or varnishes prior to polymerization. Only a small portion of the monomeric acrylic acid that is prepared is directly polymerizedxe2x80x94to, say, xe2x80x9csuper absorbersxe2x80x9d. Whereas monomers of high purity are generally required in the direct polymerization of acrylic acid, the purity requirements of acrylic acid are not so high if it is esterified prior to the polymerization.
It is well known that acrylic acid can be prepared in two stages via acrolein at temperatures ranging from 200xc2x0 C. to 400xc2x0 C. by heterogeneously catalyzed gas phase oxidation of propane using molecular oxygen over catalysts existing in the solid state (cf, eg, DE-A-1 962 431, DE-A-2 943 707, DE-C-1 205 502, EP-A-0 257 565, EP-A-0 253 409, DE-A-2 251 364, EP-A-0 117 146, GB-B-1 450 986 and EP-A-0 293 224). In this case oxidic multicomponent catalysts are used, eg, those based on oxides of the elements molybdenum, chromium, vanadium, or tellurium.
DE-C-2 136 396 reveals that it is possible to separate the acrylic acid from the reaction gases obtained in the catalytic oxidation of propane or acrolein by countercurrent absorption with a mixture of 75 wt % of diphenyl ether and 25 wt % of diphenyl. Furthermore DE-A-2 449 780 reveals the method of cooling the hot reaction gas by partially evaporating the solvent in a direct condenser (quenching apparatus) prior to countercurrent absorption. A problem arising here and in other process steps is the occurrence of solid materials in the apparatus, which reduces the availability of the plant. As stated in DE-A4 308 087 the amount of this solid material can be reduced by adding a polar solvent such as dimethylphthalate to the relatively nonpolar solvent mixture of diphenyl ether and diphenyl. (diphyl) in an amount of from 0.1 to 25 wt %.
Other principles of process exist besides the aforementioned absorption of reaction product containing the acrylic acid into a high-boiling solution (mixture). The aforementioned process differs from the other processes in that here the acrylic acid is absorbed into a high-boiling solution (mixture) in as anhydrous a state as possible. The water is removed from the process in a separate, process stage. The other processes provide for the complete condensation of acrylic acid and the water of reaction also formed during the catalytic oxidation. In such a case there is formed an aqueous acrylic acid solution which can be further purified via a distillation step using an agent capable of forming an azeotrope with acrylic acid (cf DE-C-3 429 391, JP-A-1 124 766, JP-A-7 118 766, JP-A-7 118 966, JP-A-7 118 968, and JP-A-7 241 885), or via an extraction stage (cf DE-A-2 164 767, JP-A-5 8140 039, and JP-A-4 8091 013). In EP-A-0 551 111 the mixture of acrylic acid and by-products that is prepared by means of catalytic gas phase oxidation is contacted with water in an absorption tower and the aqueous solution that is obtained is distilled in the presence of a solvent capable of forming an azeotrope with polar low-boilers such as water or acetic acid. DE-C-2 323 328 describes the separation of acrylic acid from an aqueous effluent liquor produced in the esterification of acrylic acid or an aqueous acrylic acid solution as is formed in the preparation of acrylic acid by oxidation of propene or acrolein, by extraction with a specific mixture of organic solvents.
A fact common to all processes that are industrially used for the preparation of acrylic acid or methacrylic acid is that any low, medium, and highboiling fractions produced consitute undesirable secondary components and thus streams of material that must be disposed of. These, at least three, streams of material represent a considerable cost-load for the process.
Thus it is an object of the present invention to provide a disposal process which solves the by-product problem relating to the production of (meth)acrylic acid economically and ecologically to optimum effect.
We have found, surprisingly, that this problem can be solved by burning the gaseous low-boilers together with the aqueous liquid stream of material comprising water-soluble low-boilers and/or medium-boilers, the high-boilers being optionally burnt with them.
Thus the invention relates to a process for the disposal of low, medium, and high-boiling secondary components that are formed during the production of (meth) acrylic acid in which gaseous readily volatile secondary components (1) are burned, low or medium-boiling secondary components (2) that are dissolved in water being added optionally together with high-boiling secondary components (3) treated with low-viscosity solvent the burning advantageously takes place in a combustion chamber equipped with one or more support gas burners, nozzle burners, or multi-fuel burners, eg, a flame evaporating burner.
In one embodiment the invention relates to a process in which the acrylic acid or methacrylic acid is prepared by
(a) catalytic gas phase oxidation of propene or isobutene and/or acrolein or methacrolein,
(b) absorption of the reaction product that is formed in stage (a) into a high-boiling solvent and
(c) separation of acrylic acid or methacrylic acid from the loaded solvent from stage (b), by distillation,
where, prior to the absorption in stage (b), a portion of the solvent is evaporate d and a portion of the remaining liquid solvent is disposed of, optionally following further solvent evaporation, as high-boiling secondary component (3), and unabsorbed reaction product remaining after the absorption of the reaction product in stage (b) is cooled,
where the aqueous condensate that is obtained, optionally following subsequent extraction, is disposed of as secondary component (2) and at least a portion of the gas stream that is obtained is disposed of as secondary component (1).